MacBook Pro menu display showing only letter "A" everywhere

It is my first time on this forum so I am very sorry if this question has already been asked. I looked through many pages on google and here and couldn't find an answer.
So here's my problem:
I had my Mac repaired (the hard drive has been changed yesterday morning) and everything seemed to work perfectly until i turned on my computer again yesterday evening. All the menu display (and "right click" menu) only shows the letter A like in the picture below :

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(besides my dock doesn't seem to be working either but it's not a big problem for now - I'm just mentioning it in case it's linked)

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